The use of macromers with pressure-sensitive adhesives (PSA) and for other applications are known. Macromers are relatively low molecular weight polymers having a functional reactive group at one or more terminals of the polymer.
U.S. Pat. No. 3,786,116 discloses an anionic polymerization technique used to make macromers of styrene, substituted styrenes, butadiene and isoprene. Graft copolymers of polyalphamethyl styrene and polystyrene with comonomers such as butylacrylate are exemplified. The role of the hard or high glass transition temperature (Tg) macromers is to improve the cohesive strength of the network through association of the hard segments. Synthesis of a graft copolymer of polypropylene with a polyisoprene macromer by coordination polymerization is also illustrated. The role of the polyisoprene is to modify the impact properties of the hard polypropylene. The use of low Tg macromers to improve the adhesion properties of a PSA to low energy surfaces is not taught or suggested.
U.S. Pat. No. 4,551,388 uses a hard poly(vinyl aromatic) macromer in an acrylic PSA composition. The role of the macromer is to provide cohesive strength to the network through the association of hard domains. U.S. Pat. No. 4,656,213 extends the '388 patent by using tackifiers and optionally a plasticizer for the acrylic phase.
U.S. Pat. No. 4,554,324 deals with a tacky PSA containing a macromer of Tg greater than 20.degree. C. The role of this macromer in the PSA is to improve the cohesive strength through the high Tg macromer domains.
U.S. Pat. No. 4,693,935 pertains to combining polysiloxane macromers with acrylic comonomers to obtain a PSA with reduced initial adhesion to surfaces. The adhesives are repositionable adhesives.
U.S. Pat. No. 4,732,808 deals with an acrylic skin adhesive containing a high Tg macromer. The role of the macromer is to improve cohesive strength.
U.S. Pat. No. 4,833,179 discloses the use of a high Tg macromer as a modifier to increase the storage stability of suspension polymer microspheres.
U.S. Pat. No. 4,851,278 discloses acrylic polymers synthesized with high Tg macromers which are then used as binders to reduce the transfer of tacky microspheres. The improvement in cohesive strength comes from the hard polystyrene domains.
U.S. Pat. No. 5,006,582 discloses the use of hard macromers based on polymethyl methacrylate in combination with acrylic monomers and with a tackifier for the acrylic phase. The resulting polymer is stated to have high shear properties.
U.S. Pat. No. 5,057,366 disclosed a PSA coated sheet material using the compositions of U.S. Pat. No. 4,454,324.
None of the above patents, all of which are incorporated by reference, contemplate the use of a saturated, low Tg, low solubility parameter macromer that is present in another low Tg acrylic matrix.
Tackification of acrylics to improve adhesion to low energy surfaces such as polypropylene or polyethylene is also known in the art.
U.S. Pat. No. 4,418,120 discloses a crosslinked PSA containing a tackifying rosin ester to give good adhesion to polypropylene. The PSA is claimed to have good elevated temperature properties. However, due to the use of rosin ester tackifiers, the PSA would be expected to have poor resistance to UV and oxidative degradation leading to discoloration and loss of properties on aging.
U.S. Pat. No. 4,726,982 discloses a tackified PSA that has good adhesion to high solids paint systems. It is a crosslinked acrylic polymer made from an alkyl acrylate ester and N-vinyl-2-pyrrolidone. The tackifiers claimed are poly(isobornyl methacrylate), pentaerythritol ester of rosin and mixed aliphatic-aromatic tackifier resins.
EP 303430 discloses UV on-web polymerization of an acrylate ester to form a PSA in the presence of tackifiers. The tackifiers are aliphatic polymer resins with number average molecular weight of about 300 to 2500 and such that they allow polymerization of the acrylic monomers. Very high adhesion to polyolefins is shown with good shear properties.
U.S. Pat. No. 4,988,742 relates to UV on-web polymerization with an acrylate ester PSA in the presence of hydrogenated rosin ester tackifying agent. The adhesive with the tackifier are stated to have a lower glass transition temperature than the adhesive polymerized without the tackifier. High adhesion to polypropylene combined with high elevated temperature shear properties are disclosed.
U.S. Pat. No. 5,028,484 also relates to UV on-web polymerization of an alkyl acrylate polymerized in the presence of a tackifier to form a PSA. The tackifying resins is a poly tert-butylstyrene with solubility parameter of about 7 to 9.5 (cal/cc)-.sup.1/2. This adhesive is claimed to lose not more than 70% of its adhesion when aged for a period of two weeks at 70.degree. C.
All of the above patents, incorporated herein by reference, deal with tackification of the acrylic phase in order to improve the adhesion properties. A number of these patents necessitate the use of a UV on-web polymerization process in order to obtain compatibility of the tackifier with the acrylic polymer. Since some use rosin ester type tackifiers, they would be expected to have poor UV and oxidative stability. None teach or suggest the use of a low Tg, low polarity macromer phase and its tackification to get high adhesion to apolar surfaces.
A number of patents exist which disclose pressure-sensitive adhesives involving acrylic polymers and elastomers. Some claim improved adhesion to painted surfaces and low temperature performance.
U.S. Pat. No. 4,243,500 discloses a noncrystallizing elastomer dissolved in an acrylic monomer containing an initiator that responds to UV radiation to induce copolymerization. UV on-web polymerization is the essential element in this system.
EP Publications 349216 and 352901 both deal with a PSA tape comprising a phase separated UV polymerized PSA layer made from acrylic monomers in which is dissolved a hydrocarbon elastomer which has at least one unsaturated segment having a Tg lower than that of the acrylic, such PSA having improved adhesion to painted surfaces and low temperature performance. Because of the presence of an unsaturated segment, this pressure-sensitive adhesive would have poor UV and oxidative stability losing adhesive properties over time. Because of the need to have unsaturation in the elastomer in order to obtain crosslinking to achieve the phase separated network, saturated elastomers such as Kraton G polymers were shown to be not useful.
U.S. Pat. No. 5,024,880 deals with a cellular phase separated membrane containing a multiplicity of small voids, such as a membrane made from UV polymerizing an acrylate mixture containing a photoinitiator, a surfactant (0.1 to 5%) and a saturated hydrocarbon elastomer with at least one segment having Tg lower than that of the acrylate mixture. In the absence of the surfactant very large regions of the acrylic copolymer devoid of the elastomer are obtained. Even with the surfactant, the size of the domains are sufficiently large to scatter light and make the tape hazy. This technique is useful only using on-web polymerization as the morphology is formed in situ during polymerization of the acrylate monomers.
The above three patents involve using UV polymerization process to obtain a two phase network. UV is essential to achieve crosslinking between the acrylic and the unsaturation in the elastomer is required in order to obtain the two phase network.
U.S. Pat. No. 4,143,972 deals with a PSA wherein the acrylic polymer is synthesized with a high Tg(&gt;20.degree. C.) macromer such as polystyrene macromer. This polymer is then mixed with an elastomer such as a SEBS polymer. These are not chemically grafted but are purely physical blends that on drying are held in place through the association of the polystyrene domains of the acrylic and the elastomer. High Tg domains in each of the two polymers is necessary to keep the two mutually incompatible polymers from completely phase separating.
None of the above patents deal with or suggest the grafting of a low Tg saturated macromer of low solubility parameter onto an acrylic backbone by conventional free radical polymerization techniques.